1. Technical Field
The present application relates to a nitride-based semiconductor element and a method for fabricating such an element.
2. Description of the Related Art
A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting element, because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors (which will be referred to herein as “GaN-based semiconductors”) have been researched and developed particularly extensively. As a result, blue-ray-emitting light-emitting diodes (LEDs), green-ray-emitting LEDs and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products (see Japanese Laid-Open Patent Publication No. 2001-308462 and Japanese Laid-Open Patent Publication No. 2003-332697, for example). A nitride-based semiconductor such as a GaN-based semiconductor (AlxGayInzN (where 0<x, 0<y, 0≦z and x+y+z=1)) has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlxGayInzN (where 0≦x, 0<y, 0≦z, and x+y+z=1) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four primitive vectors a1, a2, a3 and c, which are generally used to represent planes of a wurtzite crystal structure with four indices (i.e., hexagonal indices). The primitive vector c runs in the [0001] direction, which is called a “c axis”. A plane that intersects with the c axis at right angles is called either a “c plane” or a “(0001) plane”. It should be noted that the “c axis” and the “c plane” are sometimes referred to as “C axis” and “C plane”.
In fabricating a semiconductor element using GaN-based semiconductors, a c-plane substrate, i.e., a substrate of which the principal surface is a (0001) plane, is used as a substrate on which GaN-based semiconductor crystals will be grown. In a c plane, however, there is a slight shift in the c-axis direction between a Ga atom layer and a nitrogen atom layer to produce electrical polarization there. That is why the c plane is also called a “polar plane”. As a result of the electrical polarization, a piezoelectric field is generated in the InGaN quantum well of the active layer in the c-axis direction. Once such a piezoelectric field has been generated in the active layer, some positional deviation occurs in the distributions of electrons and holes in the active layer due to the quantum confinement Stark effect of carriers, and the internal quantum efficiency decreases. As a result, a threshold current increases in a semiconductor laser diode and power dissipation increases and luminous efficiency decreases in an LED. Meanwhile, as the density of injected carriers increases, the piezoelectric field is screened and the emission wavelength varies, too.
Thus, to overcome these problems, it has been proposed that a substrate, of which the principal surface is a non-polar plane such as a (10-10) plane that is perpendicular to the [10-10] direction and that is called an “m plane”, be used. In this case, “−” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index). As shown in FIG. 2, the m plane is parallel to the c axis (i.e., the primitive vector c) and intersects with the c plane at right angles. On the m plane, Ga atoms and nitrogen atoms are on the same atomic plane. For that reason, no electrical polarization will be produced perpendicularly to the m plane. That is why if a semiconductor multilayer structure is formed perpendicularly to the m plane, no piezoelectric field will be generated in the active layer, either. As a result, the problems described above can be overcome.
In this case, the “m plane” is a generic term that collectively refers to a family of planes including (10-10), (−1010), (1-100), (−1100), (01-10) and (0-110) planes. In this description, the “X-plane growth” means epitaxial growth that is produced perpendicularly to the X plane (where X=c or m, for example) of a hexagonal wurtzite structure. As for the X-plane growth, the X plane will be sometimes referred to herein as a “growing plane”. Furthermore, a layer of semiconductor crystals that have been formed as a result of the X-plane growth will be sometimes referred to herein as an “X-plane semiconductor layer”. And a nitride-based semiconductor element including a semiconductor layer, of which the growing plane is an X plane, will be sometimes simply referred to herein as an “X-plane semiconductor element”.
PCT International Application Publication No. 2010/113405 and PCT International Application Publication No. 2010/052810 relate to reducing the contact resistance of an m-plane semiconductor element. The entire disclosures of PCT International Application Publication No. 2010/113405 and PCT International Application Publication No. 2010/052810 are hereby incorporated by reference.